Electrodeposition of copper-tin alloys



State William H. Safranek and Charles L. Faust, Columbus, Ohio, assignors, by mesne assignments, to The City Auto Stamping Company, Toledo, Ohio, a corporation of Ohio No Drawing. Application March 14, 1955 Serial No. 494,257

3 Claims. (Cl. 20444) This invention relates to improvements in electroplating. More particularly, it relates to an improved bath and process for electrodepositing copper-tin alloys, whereby soluble alloy anodes may be used to maintain the bath in proper concentrations.

One of the problems in the prior-art plating of coppertin alloys has been the maintenance of proper concentrations of components of the bath. As copper and tin are deposited on the cathode, there is a corresponding depletion of these metals in the electrolyte. As a result, the composition of these baths is changed, thereby rendering the bath unsuitable for producing the desired plates. For satisfactory continuous operation, it is necessary that metals be added to the bath at the same rate at which they are taken out.

It is obvious that a continuous checking of the bath composition and addition of various components is costly and time-consuming. Such a controlled operation is not desirable for commercial plating operations. To overcome this problem, it would be advantageous to maintain the bath composition by use of anodes of the metal alloy being electrodeposited. In use, these anodes dissolve and thus replenish the bath.

In plating operations involving the plating of a single metal, no great problems are presented. However, when alloy coatings are to be formed, the rate of dissolution of the individual metal components must be carefully controlled to avoid changing the proportions of these metals in the bath.

Heretofore, separate anodes of copper and tin were utilized for electrodepositing copper-tin alloys. The use of separate anodes requires separate electrical circuits, and the current supplied to each anode must be separately adjusted so as to dissolve each of the anodes in the same ratio as they are deposited on the cathode. The operation of such a system requires careful and continuous control of the current in each circuit. a

It has also been proposed to alternately supply current to copper and tin anodes for short periods of time. However, in such a procedure, the anodes of one metal must be removed from the bath before anodes of the other metal are placed therein. Also, the time of each cycle and the current supplied to the anodes must be carefully regulated.

The operation of the copper-tin alloy bath is greatly facilitated by the use of alloy anodes whose composition corresponds to the composition of the deposited plate. In such an operation, the bath is automatically replenished in proper concentrations. However, heretofore no satisfactory method has been devised for dissolving coppertin alloy anodes at the proper rate. For example, alloy anodes containing 15 to 80 percent tin, balance copper, were dissolved with a current efliciency of less than 10 percent in prior-art baths. The cathode efiiciency of the same baths ranged from about 35 to 75 percent, depending on specific plating conditions. It can be seen that the rate of replenishment has been far below the rate of depletion, and the baths rapidly became inoperable for plating with a satisfactory cathode efficiency.

- It has been found that, by the practice of this invention,

atent O ice the dissolution of alloy anodes can be improved so that they dissolve with an efliciency of 60 to percent, thereby matching or slightly exceeding the cathode efliciency. It has also been found that the bath of this invention results in improved copper-tin alloy electrodeposits.

. Accordingly, it is one of the objects of this invention to provide a bath composition suitable for electrodepositing copper-tin alloys utilizing soluble anodes corresponding substantially in composition to the alloy deposits.

Another object is to provide an electrolyte suitable for dissolving copper-tin alloy anodes in electroplating solutions used for electrodepositing copper-tin alloys.

A further object is to provide a process for improving the brightness and smoothness of electrodeposited coppertin alloys.

In general, this invention comprises the addition of a halide salt to an alkaline copper-tin plating bath containing free cyanide. Examples of such baths are disclosed by Baier et al., U. S. Patent No. 2,511,395, issued June 13, 1950, and in the copeuding application of Faust and Hespenheide, U. S. Serial No. 98,642, filed June 11, 1949, now U. S. Patent No. 2,658,032, issued November 3, 1953.

The electrolyte of Baier et al. consists essentially of an aqueous solution of a soluble tin compound in stannate form, a soluble cuprocynaide salt, and a solube cyanide salt. Under the teaching of this patent, separate anodes of tin and the alloying metal must be used. Baier et a1. discloses the following bath composition for the deposition of a tin-copper alloy:

Grams per liter Broad Preferred Tin (as sodium stannate) 10-100 30-65 Copper (as sodium cuprocyam'de) 2-20 5-15 Free Cyanide (as sodium cyanide) 5-30 10-20 Rochelle salts 10-100 20-50 Frleaxe caustic sufficient to give a pH in excess of The bath of Faust and Hespenheide consists essentially of an alkali cyanide, an alkali pyrophosphate, an alkali orthophosphate, a material selected from the group consisting of glue, gelatin and hydrolysis products thereof, cuprous cyanide or halide, and stannous sulfate, halide, or pyrophosphate. Except for plating alloys containing very low percentages of tin, insoluble anodes or separate anodes of the alloying metals must be used. The following composition of matter, as disclosed in the Faust et a1.

patent, illustrates ranges of the various constituents added to the bath:

Another bath which can be used for electroplating according to the present invention has the same composition as the bath of Faust et a1. except that at least 0.5 mole per liter of a halide is present and an alkali metal or ammonium stannate is substituted for all or part ofthe stannous salt. Five grams of tin as stannate U are substituted for each gram of tin as stannous salt which is removed. The larger amount of tin as stannate is necessitated by the fact that tetravalent tin plates out less rapidly than divalent tin. The composition ranges 1 The total amount of tin must be 2 to 50 g/l. of stannous salt or the equivalent as stannate or mixtures of Stannous salt and stannate. Five grams of stannate are equivalent to one gram of stannons salt. Thus when no stannous salt is present, the permissible range of stannate is 10 to 250 g./l. and the preferred range to 75 g./1.

These baths have the same compositions, except for the presence of a halide, as baths disclosed in the Faust et al. patent after the latter have been in operation for some time. This is due to the unavoidable oxidation which takes place in baths initially containing all their tin in the divalent state. Baths containing a stannate at the outset are advantageous compared to those of Faust et al. because absence of oxidation simplifies control of plate compositions and color.

While either cupr'ous cyanide or a cuprous halide has been given as a suitable source of copper in baths of this type, cuprous cyanide is preferred. Sodium, potassium, or ammonium cyanide is a necessary bath constituent according to the present invention to form a cuprocyanide complex. In addition at least 0.5 mol per liter of sodium, potassium or ammonium halide is an essential constituent of baths according to this invention, as will be hereinafter explained.

The addition of a halide salt to copper-tin baths such as the foregoing permits the use of soluble alloy anodes having compositions comparable to that of a wide range of copper-tin alloys which may be plated. These anodes are dissolved with an efficiency of 60 to 95 percent, thereby equalling or slightly exceeding the cathode efficiencies commonly achieved in the bath. As a result, plating baths are automatically replenished in the correct amounts of copper and tin, to maintain optimum plating conditions in the bath.

bromide and iodide salts are equally effective. Very excellent results have been obtained by use of ammonium bifluoride, potassium fluoride, and potassium chloride.

The optimum amount of halide salt addition is dependent on the anode current density, bath temperature, solution agitation at the anode surface, etc. Also, the minimum concentration of halide salt for practical operation depends on the tin content of the alloy anodes. For example, for anodes containing up to about 20 percent tin, about 0.5 molar halide addition will provide satisfactory anode current efficiency. For anodes containing over 20 percent tin, 1.0 molar additions may be necessary. Greater amounts up to the solubility limit of the halide salt may be added, although such greater amounts are rarely necessary.

The presence of gelatin or glue in baths containing pyrophosphate improve the quality and appearance of the plate. Amino acids, which may be obtained by hydrolysis of proteins such as gelatin, glue and vegetable proteins, may be used in lieu of gelatin or glue.

It has further been found that dissolution of the alloy anodes is improved by periodically reversing the direction of the current so that the anodes become cathodic for short periods of time. The combination of periodic reversal of current with halide salt addition not only increases the cathode efiiciency but also improves the brightness and smoothness of the resulting plate. It is preferred to reverse the current for one second after every four or five seconds of direct plating, although satisfactory results can be obtained by other reversal and plating periods.

The addition of the halide salts and/or the periodic reversal of current depolarizes the anode surface and prevents the formation of oxygen, which is an underisable reaction product.

EXAMPLE I A copper-tin bath was prepared having the following composition:

G./l. Potassium pyrophosphate (K P O -3H O) 90.0 Potassium cyanide (KCN) 33.5 Cuprous cyanide (CuCN) 18.8 Stannous sulfate (SnSO,,) 10.8 Potassium dihydrogen phosphate (KH PO 20.0 Gelatin 2.0 Water Balance Table I shows results in using alloy anodes containing between and percent tin, balance copper, to plate a bright coating of approximately the same composition. Nine runs were made with varying halide additions as follows:

Table I Run Number Salt addition (g./l.) None None Ammonium bifiuoride (NH4HFz) 60 125 125 125 Potassium ehloride Potassium fluoride"--- pH 8. 6-9. 0 8. 5-9.0 8.5-9.0 8.0-8. 6 8. 2-8. 8 8.2-8. 8 8.2-9.8 7. 9-9.3 8. 2-8. 8 Cathode current density (amp./

sq. ft.) 45 45 45 45 30 3O 45 35 30 Anode current density (amp I sq. 5 G 12 5 5 10 36 Electrical yele 0) (0 Temperature 180 180 180 180 180 180 145 145 180 Anode eflicieney 1 (percent) 5 90 90 75 45 95 60 82 Cathode efficiency 1 (percent) 60 55 55 55 60 55 51 60 1 Based on' Cu+ and Sn.

1 Uninterrupted D. C.

3 Current periodically reversed 1 see. each Seen.

4 Unlnterrupted D. C.

The salts suitable for addition to copper and tin plating EXAMPLE II baths include halide salts' of alkali metals and of ammonia. Although chloride and fluoride salts may be preferred because of lower costs, it is to be understood that the The following bath and operating conditions were used to deposit a. coating containing 10 percent tinand percent copper, using soluble anodes of about the same composition:

Potassium pyrophosphate (K P O -3H O) g./l 90.0 Potassium cyanide (KCN) g./l. 46.0 Cuprous cyanide (CuCN) g./l 24.8 Stannous sulfate (SnSO g./l 3.3 Potassium dihydrogen phosphate (KH PO g./l 20.0 Liquid glue g./l 1.0 Water Balance pH 8.5-9.0 Temperature F-.. 145 Cathode current density amp./sq.ft 30 Anode current density amp./sq.ft 5

The cathode efiiciency was 90 percent, and it was found that the metal dissolved from the anodes was insuflicient to maintain the proper concentration of copper and tin in the plating bath.

As a result of the depletion of copper and tin in the bath, the cathode etficiency was so greatly reduced that further operation was impractical.

EXAMPLE IH 100 g./l. of potassium fluoride (KF-2H O) were added to a bath of the composition shown in Example II. Under similar operating conditions, the cathode efliciency was 74 percent and the anode efiiciency was 77 percent. This bath was operated for several months without it being necessary to make any adjustments on the concentration of copper and tin to maintain satisfactory plating conditions and results.

EXAMPLE IV A bath was prepared having the following compositions:

Sodium stannate (Na SnO '3H O) g./l 100.0 Sodium hydroxide (NaOH) g./l 15.0 Sodium cyanide (NaCN) g./l 28.5 Cuprous cyanide (CuCN) g./l 11.5 Water Balance Table II shows the results in using alloy anodes containing between 40 and 50 percent tin, balance copper, to plate a coating of approximately thesame composition, when the bath was operated with and without the addi- 1 Based on Ou+ and Sn.

2 Unlnterrupted D. C.

3 Current periodically reversed. 4 Uninterrupted D. 0.

EXAMPLE v A bath was prepared similar to that of Example III, except that 130 g./l. of potassium pyrophosphate were used. This bath was operated at a pH of 9.8 to 10.4 and with anode current densities up to amp/sq. ft. After four months of operation, the bath concentration was unchanged and plating results were constant.

EXAMPLE VI A bath was prepared similar to that of Example III, except that 5.5 g./l. of stannous sulfate were used. The alloy plated from this bath contained about percent tin and the balance copper. This bath was operated at a pH of 9.2 to 9.4 and with current densities of 20 amp./ sq.

ft. at the anode and 40 amp/sq. ft.'at the cathode, using alloy anodes having a composition of about 20 percent tin and the balance copper. The current was periodically reversed for 1 second after each 5 seconds of direct-current plating. During two weeks of operation, satisfactory anode dissolution was shown by the constant concentration of copper and tin in the bath, and reproducibility of plating results.

The pH of thebath was maintained in the range 9.0 to 9.8 by the addition of pyrophosphoric acid, H P O as required. Plating was carried out at a cathode current density of 45 amp./ sq. ft., an anode current density of 10 amp/sq. ft., and a temperature of 180 F. The alloy anodes contained 45 percent tin and 55 percent copper. The anodes dissolved at about the same rate that coppertin alloy plated on the cathodes. Current was passed continuously from anode to. cathode. A bright, smooth plate of about the same composition as that of the anodes was obtained.

EXAMPLE VIII A bath was prepared having the following composition:

. G./l. Anhydrous potassium pyrophosphate (K4P2O7) 87 Potassium cyanide (KCN) 48 Cuprous cyanide (CuCN) 26 Potassium stannate (K SnO -3I-I O) 18 Stannous sulfate (SnSO 12 Potassium dihydrogen phosphate (KH PO 12 Anhydrous potassium fluoride (KF) 54 Gelatin 1 Water Balance The pH of the bath was adjusted and continually .maintained in the range 9.7 to 10.2 by the addition of pyrophosphoric acid, H P O The alloy anodes contain 45 percent tin and 55 percent copper. Plating was carried out at a bath temperature of F., a cathode current density of 35 amp/sq. ft. and an anode current density of about 20 amp/sq. ft. The anodes dissolved efficiently, as indicated by the lack of any visible oxygen appearing at the surface of the anodes. A bright, smooth Amino acid mixture (vegetable protein hydrolysis product) having the following analysis:

l-leucine, 75%

l-isoleucine, 13% l-methionine, 8% 2 l-phenylalanine, 3% l-tyrosine, trace Water Balance 7 The pH was adjusted to 9.5 with pyrophosphoric acid. Plating was carried out at a bath temperature of 140 F. and the cathodecurrent density of 30 to 40 amp/sq. ft.

The anode current density was between and amp/sq. ft. The alloy anodes contained 17 percent tin, 83 percent copper, and a smooth, semibright alloy plate having the same composition was obtained. The copper and tin concentrations remained constant over a period of about two months of operation, indicating that the anode and cathode current etficiencies were about the same.

EXAMPLE X A bath was prepared having. the following composition:

Potassium pyrophosphate (K4P2O73H2O) 100 Potassium cyanide (KCN) 50 Cuprous cyanide (CuCN) 26' Potassium stannate (K SnO -3H O) Potassium dihydrogen phosphate (K PO 12 Potassium fluoride (KF-2H O) 100 Amino acid mixture (vegetable protein hydrolysis product) having the following analysis:

l-leucine, 75% l-isoleucine, 13% l-methionine, 8% 2 l-phenylalanine, 3% l-tyrosine, trace The pH of the bath was adjusted to 9.9. Plating was carried at a temperature of 145 F. and a cathode current density of amp/sq. ft. The anode current density was about 15 amp/sq. ft. The alloy anodes contained 18 percent tin and 82'percent copper, and a uniform smooth, bright plate of the same composition was obtained. The anodes dissolved eflicicntly as indicated by the lack of any visible oxygen gas at the anode surface.

The procedure described in this example was carried out in a bath of the above composition except for the omission of the amino acid mixture. A streaked plate which was nonuniform in both appearance and composition was obtained. The amino acid mixture was then added and the plate thereafter was bright, smooth, and of uniform composition.

As has been pointed out, the dissolution efliciency of the alloy anodes may be influenced by the anode current density. Satisfactory anode dissolution may be obtained with current densities ranging from 1 to 35 amp./ sq. ft. However, a current of 10 to 20 amp/sq. ft. is preferred in order to equalize the anode and cathode efiiciencies.

For best results, the voltage between alloy anodes and the cathode should not exceed about 4.0 volts, and preferably should range from 2.5 to 3.0 volts, for example, for a tank with the anodes spaced about 8 inches from the cathode. Tanks with a greater or a lesser spacing are operated with a correspondingly greater or lesser voltage.

The baths of this invention can be operated over a wide temperature range in accordance with individual preferences with regard to heating requirements and rate of deposition. Within the range of 120 to 180 F., the anode and cathodeefliciencies are approximately the same, with the result that the baths can be operated for very long periods of time without the necessity of adding copper or tin salts. However, it has also been found that both the anode and cathode efficiencies are slightly great- 8 er when the baths are operated at about F. than when operated at lower temperatures.

In summary, it has been found that the addition of a halide salt to an alkaline copper-tin alloy electroplating bath permits theuse of alloy anodes, thereby ex-' tending the time that the bath may be operated without the necessity of replenishing components of the bath. It has further been found that a periodic reversal of current through a bath containing the halide salt addition not only increases the cathode efliciency but also improves the smoothness andbrightness of the resulting electro-- plate.

This application is a continuation-in-part of our copending application Serial No. 366,871, filed July 8, 1953,

. i now abandoned.

What is claimed is: 1. In a process for electrodepositing a copper-tin alloy of over 15 percent tin from an electrolyte comprising a cuprous salt, a tin salt, and a cyanide selected from the group consisting of'the alkali metal cyanides and ammonium cyanide, the improvement which comprises the steps of electrolyzing said electrolyte having added thereto at least 0.5 mole per liter of a halide selected from the group consisting of the alkali metal and ammonium halides, and replenishing said electrolyte with copper and tin by dissolution of a soluble copper-tin alloy anode containing more than 15 percent tin inserted 1 in said electrolyte.

2. In a process for electrodepositing a copper-tin alloy of over 15 percent tin from an electrolyte comprising an aqueous alkaline solution of cuprous cyanide, a stannous salt selected from the group consisting of stannous sulfate, halide, and pyrophosphate, an alkali metal pyrophosphate, an alkali metal orthophosphate, a cyanide selected from the group consisting of the alkali metal cyanides and ammonium cyanide, and a material selected from the group consisting of gelatin, glue, and protein hydrolysis products, the improvement which comprises .the steps of electrolyzing said electrolyte having added thereto at least 0.5 mole per liter of a halide selected from the group consisting of the alkali metal and ammonium halides, and replenishing said electrolyte with copper and tin by dissolution of a soluble copper-tin alloy anode of over 15 percent tin inserted in said electrolyte.

3. In a process for electrodepositing a copper-tin alloy of over 15 percent tin from an electrolyte comprising cuprous cyanide, an alkali metal stannate, and a cyanide selected from the group consisting of the alkali metal cyanides and ammonium cyanide, the improvement which comprises the steps of electrolyzing said electrolyte having added thereto at least 0.5 mole per liter of a halide selected from the group consisting of the alkali metal and ammonium halides, and replenishing said electrolyte with copper and tin by dissolution of a soluble coppertin alloy anode containing more than 15 percent tin inserted in said electrolyte.

References Cited in the file of this patent UNITED STATES PATENTS 2,451,341 Iernstedt Oct. 12, 1948 2,658,032 Faust et al. Nov. 3, 1953 FOREIGN PATENTS 11,543 Great Britain May 13, 1912 of 1911 680,937 Great Britain Oct. 15, 1952 

1. IN A PROCESS FOR ELECTRODEPOSITING A COPPER-TIN ALLOY OF OVER 15 PERCENT TIN FROM AN ELECTROLYTE COMPRISING A CUPROUS SALT, A TIN SALT, AND A CYANIDE SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METAL CYANIDES AND AMMONIUM CYANIDE, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF ELECTROLYZING SAID ELECTROLYTE HAVING ADDED THERETO AT LEAST 0.5 MOLE PER LITER OF A HALIDE SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METAL AND AMMONIUM HALIDES, AND REPLENISHING SAID ELECTRLYTE WITH COPPER AND TIN BY DISSOLUTION OF A SOLUBLE COPPER-TIN ALLOY ANODE CONTAINING MORE THAN 15 PERCENT TIN INSERTED IN SAID ELECTROLYTE. 